In refractive index imaging, a pattern of varying refractive indices, commonly referred to as a phase hologram, is created within the material used to record the image. Holograms formed by directing a reference beam and an object beam of coherent light to enter the recording medium from opposite sides, so that the beams travel through the medium in approximately opposite directions, are known as "reflection holograms". Intersection of the object and reference beams in the recording medium forms interference fringes of material having varying refractive indices. The interference fringes lie in planes approximately parallel to the plane of the recording medium, and reflect light having approximately the same wavelength that was used to create the fringes. Hence, the hologram is viewed in reflection.
A variety of materials have been used to record volume holograms. Among the more important are: silver halide emulsions, hardened dichromated gelatin, photorefractives, ferroelectric crystals, photopolymers, photochromics and photodichroics. Characteristics of these materials are given in Volume Holography and Volume Gratings, Academic Press, New York, 1981, Chapter 10, pp. 254-304 by L. Solymar and D. J. Cook.
Dichromated gelatin is currently the material of choice for making reflection holograms due to its high resolution, and high values of refractive index modulation (i.e., high diffraction efficiency and wide bandwidth). However, dichromated gelatin has poor shelf life and requires wet processing after the material has been imaged to contain a reflection hologram. Due to its poor shelf life, the material must be freshly prepared shortly before imaging or prehardened gelatin must be used, which reduces image efficiency. Wet processing introduces an additional step in preparation of the hologram, and causes dimensional changes in the material as it swells, then shrinks, during processing. These dimensional changes affect spacing of the interference fringes. Thus, it is difficult and time consuming to reproducibly make high quality reflection holograms with dichromated gelatin.
Substantially solid, photopolymer films have heretofore been proposed for use in making holograms. U.S. Pat. No. 3,658,526 to Haugh, for instance, discloses preparation of stable, high resolution transmission holograms from solid, photopolymerizable films by a single step process wherein a permanent refractive index image is obtained by a single exposure to a coherent light source bearing holographic information. The holographic image thus formed is not destroyed by subsequent uniform exposure to light, but rather is fixed or enhanced.
More recently, excellent photopolymer systems for recording reflection holograms have been developed, as described hereinafter. As with all systems for recording reflection holograms, these substantially solid photopolymer systems reflect light having approximately the same wavelength as that used to record the hologram.
The most convenient source of coherent light to record holograms is a laser, which emits a narrow waveband of light at fixed wavelengths. For example, a krypton laser emits (red) light having a 647 nm wavelength, a helium/neon laser emits (red) light having a 633 nm wavelength and an argon laser emits (blue-green) light having a 488 or 514 nm wavelength. It may be desired to shift the wavelength of light reflected by the hologram to a different wavelength. Such a shift is achieved with prior art reflection holograms imaged in dichromated gelatin, silver halide or photopolymer film by immersing or covering the surface with a liquid solvent, which is absorbed into the matrix, swelling the hologram, and thereby causing a shift (i.e., an increase) in the reflected wavelength. This method of shifting the reflection wavelength requires liquid processing, which is both messy and difficult to control in large production runs. Thus, there is a need for an improved process to shift the reflected wavelength of reflection holograms in general, and in particular for the substantially solid photopolymer holographic recording systems described herein.